Measurements of the absolute energy spectra of cosmic-ray positrons and electrons above 7 GeV

Abstract

Abstract.A measurement of the energy spectra of cosmic-ray positrons and electrons was made with a balloon-borne magnetspectrometer, which was flown at a mean geomagnetic cut-off of 4.5 GV/c. The observed positron flux in the energy range 7-16 GeV is approximately an order of magnitude lower than that of electrons, as measured in other experiments at various energies. The power law spectral index of the observed differential energy spectrum of electrons is −2.89 ± 0.10 in the energy interval 7.5-47 GeV. For positrons the overall fit of the available data above 7 GeV has been considered. The spectral index is found to be −3.37 ± 0.26 and the fraction of positrons, e + /(e + + e − ), has a mean value of 0.064 ± 0.003. The world data on e + /(e + + e − ) from 0.1 to 30 GeV indicate that a plerion type electron spectrum is preferred over the other types. The trend of the presently existing high energy data also suggests a possible contribution of positrons produced at the pulsar polar cap. High resolution experiments capable of identifying positrons at least up to 100 GeV with high statistics are required to pinpoint the origin of both electrons and positrons in the cosmic radiation.

Constraints on cosmic-ray positron excess and average pulsar parameters

“…These authors assumed an identical effect on e + and e − fluxes for the solar modulation (defined by the modulation parameter, φ, only) and evaluated the drift effect separately. However, e + and e − would suffer the same modulation in the case of an identical energy spectrum shape, which is not the case (see for example Grimani et al 2002). Moreover, the top curve in Fig.…”

“…These observations generated many different speculations about the extra sources of positrons (see for example, Grimani 2004 and references therein). Due to the improvement of particle detectors and data analysis techniques, e + and p measurements carried out during the past fifteen years present lower values compared to the past (Grimani et al 2002;Hof et al 1996;Asaoka et al 2002). At present, there is general agreement about e + and p essentially having secondary origin.…”

Galactic secondary positron flux at the Earth

“…Since τ E determines the positron propagation scale, the related Table 1. Data are taken from CAPRICE (Boezio et al 2000), HEAT (Barwick et al 1997), AMS (Aguilar et al 2007;Alcaraz et al 2000) and MASS (Grimani et al 2002). uncertainties should have a far stronger effect on the primary contributions with a far greater spatial (or time) dependency, such as for instance nearby astrophysical or exotic point sources.…”

“…Positrons are drifted away from the Galaxy and their flux at the Earth is depleted. We note that diffusive reacceleration and Galactic convection were included separately by Lionetto et al (2005) in (Boezio et al 2000), HEAT (Barwick et al 1997), AMS (Aguilar et al 2007;Alcaraz et al 2000) and MASS (Grimani et al 2002). their prediction of the positron spectrum, with the net result of either overshooting (diffusive reacceleration) or undershooting (galactic convection) the data. If we now incorporate both processes and add the various energy-loss mechanisms, we derive the dotted curve, which also contains a bump, although of far smaller amplitude.…”

“…If we had considered other fit parameters that were also consistent with the data, by modifying for instance the normalizations so as to ensure that they remained correlated with the spectral indices 2 , we (Boezio et al 2000), HEAT (Barwick et al 1997), AMS (Aguilar et al 2007;Alcaraz et al 2000), MASS (Grimani et al 2002) and PAMELA (Adriani et al 2008). would have found quite different results opening up more or less the banana shape characterizing our predictions in Fig. 12.…”